Re: [PATCH] Add support for multiple MSI on x86

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, 4 Mar 2011, Micha Nelissen wrote:
> Jesse Barnes wrote:
> > On Sun, 13 Feb 2011 21:25:21 +0100
> > Micha Nelissen <micha@neli.hopto.org> wrote:
> > 
> > > Patch is based on earlier patch from Matthew Wilcox.
> > 
> > I like this before when Matthew was working on it too
> 
> If I understood Thomas' concerns well, it sounds like this feature is
> inherently incompatible with big SMP systems. Therefore it has no chance of
> ever being accepted into mainline in whatever form?

First of all MSI is replaced by MSI-X which does not have the
shortcomings of MSI vs. the vector allocation.

Though if we can come up with a scheme which does not create nested
loops over possibly hundreds of CPUs with interrupts disabled and
global locks held, I have no objections.

One possible solution would be to reserve a block of vectors on all
cpus at boot time via a command line option for MSI block
allocations. That would simply use a bitmap protected by a mutex to
lookup a free vector space. That would avoid the whole loop issue and
work for most of the systems which need to deal with such MSI multi
vector devices. Warning: Just an idea :)

Thanks,

	tglx